1. Field of the Invention
This invention relates to sensors, including temperature sensors.
2. Introduction to the Invention
A wide variety of electronic components and other articles are subject to damage if exposed to elevated temperature. It is, therefore, often important to be able to determine if a component has been subjected to such temperature. Various detection techniques, e.g. thermochromic materials which change color when exposed to a specific temperature, have been proposed for this purpose. Such techniques suffer from the requirement that the article must be visible in order to detect the color change, and thus are ineffective when the article is enclosed. Various electronic detectors, designed to identify an electrical change resulting from a high temperature, have also been proposed. Such detectors may not be able to determine whether a particular part of an article has been exposed to a high temperature, relying instead on the average over the entire surface. In addition, sensors which are able to maintain direct contact with the substrate, even when the substrate is not flat, are desirable. Such sensors would have sufficient flexibility that they could provide two-dimensional sensing over a large surface, and be able to be bent over an edge to provide three-dimensional sensing.
Conductive polymer compositions exhibiting a positive temperature coefficient of resistance (PTC) effect are well known. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler. At low temperatures the composition has a relatively low resistivity. However, when the composition is exposed to a high temperature, due for example, to a high current condition, the resistivity of the composition increases, or xe2x80x9cswitchesxe2x80x9d, often by several orders of magnitude. The temperature at which this transition from low resistivity to high resistivity occurs in a PTC composition is the switching temperature, TS. TS is defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of an element prepared from the composition as a function of temperature which lie on either side of the portion of the curve showing a sharp change in slope. Similarly, a composition exhibiting a negative temperature coefficient (NTC) of resistance will have a switching temperature, TS, in the region at which the resistivity goes from a high to a low value.
The use of a sensor comprising a PTC conductive polymer to detect an overtemperature condition is known. For example, Japanese Patent Application No. 10-95019, filed Apr. 7, 1998 (K. K. Raychem), the disclosure of which is incorporated herein by reference, discloses a elongate temperature sensor which can be used to detect overheating in a battery. Batteries which overheat are subject to damage, and in addition may damage the packaging surrounding them and the components in contact with them. While overheating may be due to external environmental conditions, for secondary, i.e. rechargeable batteries, such overheating may occur as a result of excessive charging. The overheating may result in damage to the internal components of the battery, the generation of gas, and, under extreme conditions, explosion of the battery. For example, for nickel-metal hydride batteries, it is desirable to keep the temperature below 100xc2x0 C. to avoid the evolution of hydrogen. It is, therefore, important to identify batteries which have been subject to overheating before damage can occur. In Japanese Patent Application No. 10-95019, a sensor is attached to a plurality of batteries. An elongate tape composed of a PTC conductive polymer comprising spaced-apart sensing components and connecting components is in contact with the individual battery cells. The sensing components are electrically connected in series so that the resistance of the sensor is the sum of the resistances of each individual sensing component. The sensor is positioned so that a sensing component is in contact with the external surface of a battery cell, and preferably each individual battery cell contacts a different sensing component. When the battery cells are in a normal, low temperature condition, the resistance of the sensor is low. If, however, one battery cell heats to a temperature above TS, the resistance of the sensing component in contact with that battery cell increases, thus increasing the total resistance of the sensor and indicating that at least one battery has been subject to overheating.
The approach taken in Japanese Patent Application No. 10-95019 requires that the entire battery cell heat to a temperature sufficient to cause the PTC conductive polymer composition to switch. This means that if there is a relatively small hot spot inside the battery cell, which is sufficient to cause damage to a small region of the battery but is insufficient to heat the entire cell, it will not be detected. Many batteries, such as lithium ion polymer batteries have a layered sheet construction in which an anode and a cathode are separated by a separator, and in addition comprise an electrolyte. In practice, the layered sheet is rolled into a cylinder and positioned inside a can to form a battery cell. A hot spot in the center of the cylinder, due, for example, to inhomogeneities in the anode, cathode, or separator, can cause damage to the electrolyte, which is solvent-based. It is, therefore, desirable to have a sensor which can detect not just the temperature of the entire battery cell, but rather the temperature of individual spots within the battery cell.
In another application, a lithium ion polymer battery, used unrolled in its thin, flat configuration, can be positioned behind the screen of a laptop computer to detect temperature changes. For this application, it is necessary to have an array of sensing elements as a point sensor applied to one part of the screen may not reflect a change elsewhere on the screen.
Detecting individual spots on a substrate is also important for articles other than batteries. It is desirable to have a sensor in which the pattern of the sensing elements can be designed for a specific configuration, so that individual components, e.g. individual elements on a printed circuit board, can be in contact with the sensor. Such a sensor can be used for situations in which the temperature at one spot is not representative of the entire surface, but for which sensing is still required. Furthermore, it is desirable to have a sensor which can be used to detect hot spots over two dimensions and over a large area. We have now found that a laminar sensor comprising a laminar sheet comprising a conductive polymer composition and a plurality of sensing elements has sufficient flexibility to contact substrates of nonuniform or irregular structure, as well as the ability to detect temperature changes over a broad area. In addition, the sensor can be used to detect resistance changes resulting from pressure or exposure to solvents. Thus, in a first aspect this invention provides a laminar sensor for detecting changes, e.g. temperature changes, on a laminar substrate, the sensor having a resistance at 20xc2x0 C. RT and comprising
(1) a laminar sheet which (a) has a first surface and a second opposite surface, and (b) comprises a conductive polymer composition which (i) exhibits temperature dependent resistance behavior and (ii) has a switching temperature TS;
(2) a plurality of sensing elements and (a) each of which comprises an electrode pair, said electrode pair comprising a first electrode and a second electrode, said electrodes being separated from each other and in contact with the laminar sheet, and (b) which are electrically connected in a resistive network, at least some of said sensing elements connected in series; and
(3) two electrical leads for connecting the sensing elements into a circuit.
In a second aspect, the invention provides a lithium ion polymer battery which comprises
(A) a laminar battery element surrounded by an insulating material, said battery element comprising (1) first and second battery electrodes, (2) an anode, (3) a separator, (4) a cathode, and (5) and electrolyte; and
(B) a laminar temperature sensor of the first aspect of the invention positioned in direct contact with the insulating material and covering at least 75% of one laminar surface of the insulating material.
In a third aspect, the invention provides an electrical circuit which comprises
(A) a laminar sensor of the first aspect of the invention; and
(B) detection equipment electrically connected to the electrical leads to detect a change in the sensor.